1. Field of the Invention
The present invention relates to a semiconductor light emitting device formed from II-VI compounds and, more particularly to a semiconductor light emitting device as a light emitting diode and semiconductor laser capable of green or blue light emission.
2. Description of the Related Art
Recently there has been a considerable interest in the II-VI compound semiconductor light emitting device as a semiconductor light emitting device capable of green or blue light emission. It is composed of an n-type cladding layer, of mixed crystal of ZnMgSSe, an n-type guide layer of ZuSe, an active layer, a p-type guide layer of ZnSe, and a p-type cladding layer of ZnMgSSe, which are formed sequentially on top of the other on a substrate by molecular beam epitaxy (MBE). The p-type cladding layer and p-type guide layer are doped with nitrogen (N) as a p-type impurity by exposure to a nitrogen plasma generated by an RF (radio frequency) plasma generator or an ECR (electron cyclotron resonance) plasma generator.
Doping with nitrogen as a p-type impurity as mentioned above suffers the disadvantage that the carrier concentration in the p-type cladding layer and p-type guide layer increases to a certain level in proportion to the amount of doped nitrogen, whereas the carrier concentration does not increase any longer and remains unchanged once it has reached a certain level even though the amount of doped nitrogen is increased and it rather decreases when the amount of doped nitrogen is increased further. The carrier concentration which has reached a certain level is called saturated carrier concentration.
Incidentally, the saturated concentration of activated nitrogen is empirically known to be about 1.times.10.sup.18 cm.sup.-3 for ZnSe and about 2.times.10.sup.17 cm.sup.-3 for znMgSSe mixed crystal. There is a relationship as shown in FIG. 3 between the concentration of doped nitrogen and the concentration of activated nitrogen (N.sub.A -N.sub.D). The result of doping nitrogen in excess of the saturated concentration of activated nitrogen is point defects of every kind due to inactive nitrogen. These defects lead to carrier compensation and inactivation which in turn leads to a decrease in the concentration of activated nitrogen.
Moreover, inactive nitrogen appears to deteriorate and adversely affect the active layer and lower the light intensity. This is a hindrance to long-lived semiconductor light emitting devices.